Only two materials, CaF.sub.2 and fused silica, are currently available for refractive optical components such as lenses for ultraviolet optical systems operating in the wavelength range of about 193 nm which is the nominal wavelength of the ArF excimer laser. CaF is a crystal and is much more difficult to form into optical components than fused silica which has an amorphous molecular structure.
A serious problem associated with fused silica is that various types of radiation including UV radiation will cause it to undergo "densification" or "compaction". This is a serious problem for the makers of stepper and scanner equipment which uses ArF lasers as a light source for integrated circuit lithography. These machines use very high precision lenses to condition the laser beam for printing circuits on silicon wafers. Compaction in the range of a few parts per million for imaging of a photo mask can seriously degrade performance of these lenses.
The densification of fused silica with radiation has been studied thoroughly. See for example N. F. Borrelli, et al., "Densification of fused silica under 193-nm excitation", J. Opt. Soc. Am. B/Vol. 14, No. 7/July 1997 and the many papers cited therein. That study found "that the derived densification follows a universal function of the dose, defined as the product of the number [N] of pulses and the square of the fluence [I] per pulse". FIG. 14 of that report has been reproduced here as FIG. 1. This and other prior art reports suggest methods of reducing the rate of densification such as by decreasing the energy per pulse and increasing the number of pulses.
What is needed is a better method of preparing fused silica to extend the useful life of fused silica optical elements used with high energy ultraviolet light sources.